The invention provides improved lateral FET structure for bidirectional power switching, including AC application. The invention particularly provides enhanced gating capability by means of trapped charge on floating gate electrode means proximate and insulated from a pair of FET channels each conducting current in both directions.
Copending application Ser. Nos. 390,719 and 390,479, filed June 21, 1982, disclose AC power FET structure including laterally spaced source regions and channel regions having a common drift region therebetween. Upon application of voltage of either polarity across main terminals connected to the source regions, current flow in either direction is controlled by the potential on gate electrode means proximate the channels. The potential of the gate electrode means produces electric fields of sufficient intensity to invert the conductivity type in the channel regions, enabling bidirectional conduction.
A need has arisen in some implementations for producing electric fields from the gate electrode means without having to reference the latter to one of the main AC line terminals when applying gate potential. The present invention addresses and satisfies this need by storing trapped charge on electrically isolated gate electrode means. Charging electrode means is disposed proximate the gate electrode means for charging the latter. Electrical insulation layer means is between the gate electrode means and the first and second channel regions. Electrical insulation layer means is also between the charging electrode means and the gate electrode means, with the latter being charged by carriers tunneling through the last mentioned insulation layer means in response to given polarity potential on the charging electrode means.
The AC power FET has an OFF state in the absence of stored trapped charge on the gate electrode means, with the junction between the drift region and one of the channel regions blocking current flow toward one of the source regions, and with the junction between the drift region and the other of the channel regions blocking current flow toward the other of the source regions. Charge is depleted from the gate electrode means by tunneling back through the last mentioned insulation layer means in response to opposite polarity potential on the charging electrode means.